I had an IBM final assessment for IT Specialist last week but at the same time there were people doing Business development etc and we all did the same things so it'll be pretty much like the following (I didn't get offered a job!-so I'm not going to be the perfect guru here)
1) Group exercise- A really tough logical puzzle which is slightly like Soduku but without any numbers. Basically your given different pieces of information and as a group you have to logically work out the seating order of some people. The only way you can practice this sort of thing is to find a quiz book or something along those lines. Our group came nowhere near to solving it.
2) Communications exercise- small case study you have to read some notes a colleague has made and then try a sales pitch to your interviewer. Then you have to write an e-mail based on same thing selling it to someone else.
3) 2 interviews- 1 foundation, 1 business (former with HR person, later with person who does role you apply for), these interviews really overlap and they both asked me some of the same questions. What makes you want to work for IBM? Why do you think IBM are so ethical? who are our competitors? Name a project you've worked on? Tell me something about yourself that is non-academic/ IT related? Name a time yuo decided to accomplish something extracurricula- basically your generic compentency interview. HR women was a bit of a nutter and went off on tangents- nice though.
Anyhoo, I flunked although luckily I had another offer which I would have taken over Big Blue anyway, loads of the staff looked a bit depressed. In my application I said I wanted to work for IBM because it was a meritocracy and the HR gal became very sarcastic and basically told me that was all Bullsh*t! good luck though, none of its that hard apart from the logical puzzle thing which is a mindscrew.